In general, a polyimide (hereinafter referred to as "PI") resin denotes a high heat-resistant resin which is manufactured by condensation polymerization of aromatic tetracarboxylic acid or the derivatives thereof with aromatic diamine, or aromatic diisocyanate followed by imidization. However, the PI resin is insoluble in solvent and non-meltable by heat.
Further, the PI resin can have a variety of molecular structures depending on the types of the monomers thereof. As for the aromatic tetracarboxylic acid constituent, pyromellitic dianhydride (PMDA) or biphenyltetracarboxylic dianhydride (BPDA) are used herein. As for the aromatic diamine constituent, oxydianiline (ODA) or p-phenylene diamine (p-PDA) are used for condensation polymerization. The typical PI resin has a repetitive unit of the following formula ##STR2##
The PI resin with the formula 10 as repetitive unit is high heat-resistant, insoluble, and non-meltable with the following properties:
(1) excellent heat-resistance oxidation property; PA1 (2) superior heat-resistance based on the utilization at high temperature, i.e., 260.degree. C. for a long-term use, and 480.degree. C. for a short-term use; PA1 (3) excellent electrochemical and mechanical properties; PA1 (4) excellent radiation resistance and low temperature properties; PA1 (5) intrinsic non-combustible properties; and PA1 (6) excellent chemicals-resistant properties.
In spite of the fact that the PI resin with formula 10 as repetitive unit possesses an excellent heat resistance property, the processing therein is extremely difficult due to insolubility and non-melting property.
To overcome the shortcomings of the PI resin, several methods have been devised: a) method of introducing polar groups into the backbone or side chains of the polymer, b) method of introducing the connecting groups or pendant groups with a large volume into the polymer, and c) method of enhancing the flexibility of the backbone of the polymer.
In particular, as part of research to enhance the solubility of the PI resin, T. Kurosaki et al. reported a method of preparing a soluble PI using alicyclic anhydride as a monomer (Macromolecules, 1994, 27, 1117 and 1993, 26, 4961). Also Qn Jin et al. disclosed in 1993 a method of preparing a soluble PI resin using the cyclic diamine (J.P.S. Part A. Polym. Chem. Ed., 31, 2345.about.2351).
The soluble PI resins as modified by the aforementioned methods have witnessed better solubility due to enhanced flexibility of the chains. However, the actual use thereof seems to be problematic since the high thermal stability and mechanical characteristics, which are advantages of the conventional PI resin, are seriously affected thereby.
To overcome such shortcomings, the inventors herein have for several years conducted intensive studies to improve the solubility and melting property of the polyimide resin, a typical thermoplastic and high heat-resistance resin, for improvement in the processability of the conventional aromatic high heat-resistance polymers. As a result, the inventors have discovered that the solubility of a polymer can be significantly enhanced by introducing isophorone diamine (hereinafter referred to as "IPDA"), an aliphatic diamine compound containing trimethylcyclohexyl group, into the backbone of polymer (U.S. Pat. No. 5,521,276).
Further, the inventors have endeavored to prepare a soluble PI resin with better thermal resistance than the conventional PI resin with IPDA. As a result, the inventors have succeeded in preparing a variety of homo- and co-PIs with enhanced physical properties, using aromatic diamine compounds containing trimethylcyclohexyl group (e.g., trimethylcyclohexylidene dianiline: hereinafter referred to as "TMCH-DA") as a monomer (Korean Patent Application No. 97-2811).
Based on the above studies, the inventors have significantly increased the solubility and thermal resistance of a polymer by introducing various substituents into the cyclohexylidene group instead of a methyl group in the trimethylcyclohexylidene group, which is a connecting group between the two phenyl groups of the TMCH-DA.